I think this may be caused by an SSL certificate trust issue. I'd recommend checking the certificate used by Server over 6443. If this is a certificate that was imported into Server, can you confirm that Portal trusts the certificate authority that signed the certificate? If this is the default self-signed certificate that was generated when Server was installed, can you confirm that the hostname listed in the Server admin url matches the common name for the certificate? Another good way to test if Portal trusts the certificate is to use the checkUrl.jsp endpoint.